We have only one DisplayState, so there is no need for the "next"
linking, rip it.  Also consolidate all displaystate initialization
into init_displaystate().  This function is called by vl.c after
creating the devices (and thus all QemuConsoles) and before
initializing DisplayChangeListensers (aka gtk/sdl/vnc/spice ui).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>